All water found on the earth's surface contains more or less foreign matter. The purest is melted snow or rain water, collected at a distance from towns. The most common impurities are salts of lime, as the sulphate, and the carbonate held in solution by an excess of carbonic acid. Besides these, water always contains a certain amount of gases dissolved in it, as common air, or rather air rich in oxygen and carbonic acid. The nature of the saline impurities varies much with the kind of soil through which the water flows; for some substances, as silex, are almost insoluble, whereas limestone and gypsum dissolve to a considerable amount, the former especially, when the water is impregnated with carbonic acid. When these foreign matters exist in the water to an extent sufficient to impart a sensible taste, it is called a mineral water: these have been divided into four classes, depending on their chemical composition.

The following are the most important: -

Class 1. - Chalybeate Or Ferruginous Waters

These waters owe their efficacy to the iron contained in them; in many waters traces of iron exist, but such only are named chalybeate as possess sufficient of this metal to endow them with decided medicinal powers. In some springs, the iron exists in the form of carbonate held in suspension by excess of carbonic acid; when such are exposed to the air, peroxide of iron is soon formed and deposited: others contain sulphate of iron, often associated with sulphate of alumina, sometimes with chloride of iron.

The most important chalybeate waters, in which carbonate of iron is found, are those of Pyrmont, Spa, Schwalbach, Tunbridge Wells, and Harrowgate.

Iron exists as a sulphate in the waters of Sand Rock Isle of Wight, Brighton, etc.

Therapeutics. Chalybeate waters are useful in anaemic conditions of the system; those containing the carbonate of iron are generally preferred, agreeing better with the stomach, and being less astringent. Individuals of plethoric habits should avoid chalybeate waters.

Class 2. - Acidulous Or Carbonated Waters

These waters contain a large amount of carbonic acid, which gives them their acidity, and causes them to sparkle; they usually hold in solution carbonates of lime, soda, and magnesia, which often become deposited on exposure from the escape of the carbonic acid, which acted as the solvent. The most celebrated of these waters are those of Carlsbad, Seltzer, and Ilkestone, near Nottingham.

Therapeutics. These waters are useful in atonic forms of dyspepsia, the free carbonic acid improving the tone of the stomach; they also, from the alkaline salts they possess, act as alteratives, increasing the secretion of the kidneys and skin, and are often valuable in chronic visceral diseases, gout, rheumatism, and some calcareous affections.

Class 3. - Saline Waters

These waters contain various salts, as sulphates of soda, magnesia, and lime; chlorides of sodium, calcium, and magnesium; carbonates of lime and soda, etc. Hence they may be subdivided:

Some, containing chiefly sulphates of magnesia and soda, are named purging saline waters, as those of Cheltenham, Leamington, Epsom, Seidlitz, and Pullna.

Some, having carbonate and sulphate of lime for their principal ingredients, are called calcareous waters, as those of Buxton, Bath, and Bristol.

Others contain chiefly chlorides, with, now and then, traces of iodine and bromine, called salt waters, as those of Wiesbaden, Baden-Baden, and Kreutznach. Sea water also belongs to this division.

Lastly, some saline waters are noted for the amount of alkaline carbonates they contain, these are termed alkaline waters, as those of Vichy and Ems, etc.

Therapeutics. The purging waters are indicated in cases where congestion of the portal system is present, whether from organic visceral disease or not; the calcareous waters in chronic gout and rheumatism, and some skin affections: they act as powerful stimulants and alteratives, increasing the urinary and cutaneous excretions; the simple saline waters are more adapted for scrofulous affections, as glandular enlargements, etc.; and the alkaline waters in gout, and urinary diseases connected with excessive formation of uric acid, as they tend to induce a less acid condition of urine.

Class 4. - Sulphuretted Or Hepatic Waters

All contain sulphuretted hydrogen in solution, and are readily known by their odour; the most celebrated are those of Harrow-gate, Moffat, Cheltenham, Aix-la-Chapelle, Borcet, and Aix, in Savoy: some contain an alkaline sulphuret as well as sulphuretted hydrogen.

Therapeutics. These waters act as stimulants, especially on the skin and uterine system, and are used chiefly in chronic skin diseases, certain cases of chronic rheumatism, and uterine affections, etc.